The growth of a distinctive medieval tradition of constitutional thought was a complicated, difficult process. That is not surprising. Western constitutionalism itself is an unusual phenomenon in the general history of human government. In the past, when primitive peoples emerged from tribalism to form a civilization and a state, they most commonly turned to theocratic absolutism as the only effective way of maintaining order and unity in a complex society – to the rule of a sacred monarch, a priestking, a divine emperor. So too, during our present era, the new ‘thirdworld’ nations, which everywhere began with brave dreams of democracy, have almost everywhere found it necessary to accept some form of dictatorship in order to survive. The classical age provides examples of city-states that learned to practice self-government within the framework of a small-scale society; but the Greek cities were eventually assimilated into a Roman state that moved from avowed republicanism to military dictatorship to overt theocracy. Humans find it consoling to imagine that the order imposed by their rulers reflects a divine ordering of the universe; most of the time, as Bernard Shaw observed, ‘The art of government is the organization of idolatry.’ (The great advance of the twentieth century has been our discovery that it is possible to combine all the advantages of theocracy with all the conveniences of atheism.) I do not suggest that this is an inevitable outcome of human affairs – merely a statistical probability. The historical problem of how constitutional theories and practices could first emerge and persist is a fascinating one partly because the practical problem of whether constitutionalism can survive and expand in the modern world remains so delicately poised.